The invention relates to an apparatus for non-contact blade vibration measurement having sensors which are arranged around the circumference of a rotor which is formed with rotor blades, having a signal detection unit and having an evaluation unit, as well as a method for non-contact blade vibration measurement.
Basically, non-contact blade vibration measurement [berührungslose Schaufelschwingungsmessung (BSSM)] on rotor blades of axial compressors of gas turbines for aircraft propulsion or for stationary industrial gas turbines is known from the prior art in the international arena under the name of tip timing. These measurements are required because great static and alternating aerodynamic forces are acting on the blade during operation, which excite the blade to vibrate. Among other things, the pressure distribution in the gas flow because of the intake geometry, changes in the clearance between the blade tips and the housing caused by oval deformations of the housing, or the stator grille can be responsible for this. These sources of vibration are linked to the engine housing, and therefore generate vibrations with multiples of the rotational speed of the rotor shaft, so-called resonant vibrations. Other types of vibrations are triggered by aerodynamic instabilities, and flutter vibrations and vibrations with compressor pumps belong in this category.
Strong vibrations may lead to the formation of cracks in the compressor blades and ultimately to the failure of blades, which can bring about the total failure of a turbine engine. This can have fatal consequences, particularly in the case of components for air traffic.
As a result, blade vibration measurements are conducted during the development of these types of engines. Corresponding measurements in series production engines are just as possible in principle. In the case of older known measuring methods, the blade vibrations are monitored by means of strain gauges and telemetry transmission, i.e., not in a non-contacting manner. However, this is associated with increased equipment expenses and corresponding costs, which is why the non-contact measuring methods are preferred.
Non-contact blade vibration measurement systems are therefore used to monitor blade vibrations in compressors on the blade tips and are able to reliably predict the service life of the rotor blades, wherein the method can, to some extent, also simultaneously measure the radial clearance between the blade tips and the housing. Different measuring systems are known in this case.
A known measuring system for non-contact blade vibration measurement uses, for example, capacitive sensors in the engine housing to measure both the vibrations as well as the radial clearance on the rotor blades of axial compressors. In this case, the sensors used can be deployed in a temperature range of up to approx. 700° C., which makes use on all compressor stages possible. The basic principle of the vibration measurement is a run-time measurement of the blade tips passing under the sensors. In the process, vibrating blades pass by the sensors sooner or later depending on the momentary deflection state. The clearance information in this measuring method comes from the variation of the signal amplitude, when a blade approaches a sensor and passes under it. The vibration frequencies, vibration amplitudes, and radial clearances of all rotor blades of a rotor stage can be determined and analyzed by analyzing the passing times and the amplitude swing.
For the vibration measurement, the precise positions of the sensors on the circumference of the housing must be known, something that can be accomplished in an idle state by simply taking measurements of the respective sensor position. However, radial movements of the rotor axis relative to the housing axis and to housing deformations occur when the engine is in operation. The causes of this, among other things, are the effects of gravity, asymmetric conditions at the engine intake or thermal effects, such as those that are triggered when opening and closing bleed valves, for example. These types of effects lead to changes in the effective sensor positions as viewed from the rotor and therefore to interference or mistakes in the vibration analysis. This is particularly the case when analyzing resonant vibrations of higher lying vibration modes, which generate only small deflections on the blade tip. At the latest, when the displacement of the effective sensor position reaches the order of magnitude of the vibration deflection being measured, a precise measurement of the vibration amplitude is only possible with great difficulty or not at all.
The effect of the described interference is critical, above all, for resonant vibrations, because in this case, at a specific rotational speed, the rotor blades pass by the sensors during each rotation in the same deflection state, because the vibration movement is bound in terms of phase to the engine housing.
These effects are not taken into consideration with known tip-timing systems. Resonant vibrations are evaluated as a rule rotationally. The cited interference, i.e., the movement of the rotor axis relative to the housing axis and the housing deformation, i.e., in particular the oval deformations, are superimposed, however, on the measurement data that is obtained in this manner and restrict the amplitude resolution or the measuring accuracy accordingly.
In some cases, the so obtained measurement data of a complete resonance pass are analyzed jointly by resonance curve fit. Fit parameters are amplitude and frequency at the resonance point, as well as other parameters.
This largely eliminates the effect in the range of the resonance of constant offsets on the angle positions of the sensors, but when there is a sudden change in the resonance in the rotor position or the housing shape, such as can occur, for example, by opening or closing the bleed valves, this appears as interference or an error in the measurement data.
As a result, the invention is based on the objective of avoiding the technical problems of the prior art that was cited above, and providing an improved apparatus and an improved method for non-contact blade vibration measurement. In particular, one objective of the present invention is eliminating the effect of rotor radial movements and housing deformations, the so-called oval deformations, on the measurement data, thereby ensuring high amplitude resolution for the vibration analysis under all conditions.
The invention avoids the technical problems of the prior art and provides an improved apparatus and an improved method for non-contact blade vibration measurement. In particular, the inventive attainment eliminates the effect of rotor radial movements and housing deformations, i.e., oval deformations, on the measurement data, thereby ensuring high amplitude resolution for the vibration analysis under all conditions.
The inventive apparatus for non-contact blade vibration measurement having preferably capacitive sensors which are arranged around the circumference of a rotor which is formed with rotor blades, having a signal detection unit, and having an evaluation unit is characterized in that the apparatus features devices to determine the rotor position and/or the housing deformation, i.e., the oval deformations. The devices to determine the rotor position and/or the housing deformation may be formed advantageously both as hardware components or as software components. In this case, it is especially advantageous if the characteristic of the existing sensors for clearance measurement is utilized for the non-contact blade vibration measurement sensors and the corresponding measuring results are used to determine the rotor position and/or the housing deformation. This represents a simple and cost-effective solution. Alternatively, additional position sensors could also be used for determining the shaft position or the sensor position on the housing.
In order to calculate the rotor position, at least three sensors for clearance measurement must be arranged on the housing in a distributed manner over the circumference. In order to calculate the rotor position and housing deformation, i.e., for the so-called oval deformations, at least five or more sensors should be available. If it is possible to specify a main direction of the oval deformations, for example, due to the type of engine mount, it is also possible to work with only four sensors and, despite this, calculate the extent of the ovality. Alternatively, it is possible to calculate an ovalization with only four sensors under the assumption of an overall minimal deformation of the housing.
The inventive method for non-contact blade vibration measurement having sensors which are arranged around the circumference of a rotor which is formed with rotor blades, and having a signal detection unit, and having an evaluation unit features the following steps:
a) Detecting the sensor signals, in particular by means of the signal detection unit, in the evaluation unit;
b) Analyzing the passing time and the amplitude swing;
c) Analyzing the radial clearance;
d) Calculating the rotor position and, if applicable, the housing oval deformation on the basis of the radial clearance at the respective sensor position;
e) Calculating the effective sensor positions for the rotor; and
f) Analyzing the vibrations on the basis of the effective sensor positions.
In the case of the inventive method, the sensor signals, for example, are read directly into an evaluation unit via a signal detection card, which converts the analog measuring signals into digital signals, and analyzed there with respect to the passing time and amplitude by means of appropriate hardware and/or software. Afterwards, the analysis regarding the radial clearance and vibrations of the rotor blades takes place.
Using the evaluation software, it is possible to continuously calculate the position of the rotor axis and deformation of the housing from the radial clearances, which are present at each sensor position. The sensor positions effective for the rotor can be calculated from these data. And from these, correction valves for the nominal positions of the sensors may in turn be determined. These corrections or this “tracking” of the sensors take place in the same time period as the vibration analysis, so that the sensor positions can be tracked point for point via the time or via the rotational speed. As a result, the interference from the movement of the rotor axis relative to the housing axis and from the housing deformation can be eliminated.
Because of these inventive measures, the amplitude resolution can be substantially improved in the analysis of resonant vibrations. In particular, in the case of the real-time analysis, vibration amplitudes should be indicated immediately and even in the case of a constant rotational speed, which is why a resonance curve fit known from the prior art is not possible.